It is common to compression mold plastic articles including closures, as contrasted to injection molding plastic articles. Typical patents comprise U.S. Pat. Nos. 2,072,536, 2,155,316, 2,218,456, 2,402,462, 2,891,281, 3,210,805, 4,296,061, 4,314,799, 4,343,754, 4,355,759, 4,497,765, 4,640,673, 4,755,125 and EP 0 091 653 A2.
In the compression molding of plastic articles, there are inherent variations that can affect the resultant articles. One such variance is the manufacturing tolerance applied to the tools. Accordingly, the molding sets on a machine are not identical. Thus, when the tools are made up in the molding position, the volume of the space between the molding surfaces varies between mold sets. A further variance is the weight and/or volume of the plastic charge that is placed within each mold set.
In the above-identified patent application Ser. No. 08/473,479 filed Jun. 7, 1995 and U.S. Pat. No. 5,554,327, incorporated herein by reference, there is disclosed an invention which provides a method and apparatus for compression molding plastic articles including closures wherein the forming pressure can be accurately controlled; wherein the forming pressure can be readily adjusted; wherein lateral forces on the tooling are not applied directly to the forming tool; wherein the tooling may be readily replaced; wherein the number and size of tool stations can be readily changed; and wherein various kinds and sizes of articles including closures can be readily made by changing the tooling and associated actuating mechanisms; wherein the tooling will compensate for variations in pellet or charge weight, variations in mold tooling volume in the closed mold position and wherein a substantial overload such as a double charge of plastic can be readily absorbed without overloading the tooling or the overall apparatus.
In the aforementioned patent application Ser. No. 08/473,479, the method and apparatus for compression molding plastic articles including closures includes providing co-acting sets of tools including a first set for moving a core and core sleeve into engagement with a cavity mold relative to a second set of tooling. The first set of tooling includes an actuator between the tooling and a fixed upper cam. The second set of tooling includes an actuator supporting the cavity mold and associated with a lower fixed cam. A gas cylinder charged with atmospheric gas at a predetermined pressure, preferably nitrogen, is provided in the second set of tooling and controls the compression molding force. In a preferred form, a plurality of sets of tooling are provided in circumferentially spaced relation on a rotating turret supported by a central column. A common manifold supplies the pressure at accurately controlled pressure to each of the nitrogen cylinders.
In the above described compression molding machines, the individual tooling assemblies within the array of tooling, are each capable of absorbing excessive tooling strokes up to approximately i inch whereas the normal deflection of the tooling is in the order of approximate 0.030 inch. During normal operating conditions, an occasional overload such as a double pellet, can be adequately absorbed by the nitrogen cylinder without exceeding the limiting mold force controlled by the system nitrogen pressure. However, a slow build up of cured resin with the tool, can eventually fully extend the stroke of the nitrogen cylinder, thus negate the force limiting characteristic of the nitrogen cylinder. Further, a foreign body inadvertently introduced into the cavity can immediately cause a tooling overload as the tooling is closed by the opposing fixed cams.
Among the objectives of the present invention are to provide a method and apparatus for providing overload protection for compression molding machines; which prevents damage to the machines; which may include catastrophic overload protection; and which may include predictive overload detection.
In accordance with the invention, a movable top plate supports a rigid cam including a cam profile. At least one nitrogen cylinder acts downwardly onto the top plate to maintain the top plate and cam profile in a fixed position during normal machine loading. A support bracket supports the nitrogen cylinder and is secured to the base of the machine by a support leg or by additional supports connected to the machine base. During normal operation, the normal variations in pellet size are compensated for by the gas cylinder within the second or lower set of tooling as shown in the aforementioned patent application. If the desired predetermined maximum machine loading (force) as controlled by the nitrogen pressure to the nitrogen cylinder positioned with respect to the top plate is exceeded, the top plate lifts upwardly against the nitrogen cylinder, and thus relieves the loading. In one form, this movement is sensed by a switch and the operation of the compression molding machine is stopped to provide catastrophic overload protection. In another form, a load cell positioned at the off load position of the cam continuously monitors the forces on the cam and is used to provide a signal of a condition that could lead to catastrophic overload and to initiate action to prevent the catastrophic overload.
In the preferred embodiment, a top plate is pivoted from two supports such that it will lift away from the forming area. A tapered dowel locating system is provided in the forming area whereby the top plate can lift, and yet relocate on the tapered dowels as it re-engages after the overload is removed. Detection means are required for top plate lifting, and suitable pivoting means at the alternate supporting frame is required. Spring means are provided to retain the top plate.
In the form to prevent a catastrophic overload encountered during the forming and holding stage of the cam, the stroke of the nitrogen cylinder within the tooling is exceeded, causing a "solid condition" of the tooling stack, and the resulting force exceeds the opposing force of the nitrogen cylinder retaining the top plate in a fixed position. This force overcomes the retaining force on the top plate and cam allowing the plate to lift to accommodate the abnormal condition. Detection means signals the displacement of the top plate, and initiates a machine emergency stop and quick exhaust of the top plate retaining nitrogen cylinder. Thus, the overload condition is avoided, and the machine is secured until the overload condition is eliminated and the machine is reset for normal operation.
In the form for providing predictive overload protection, a load cell positioned adjacent the off load position of the cam continuously monitors the forces on the upper cam. In the case of excessive load, force is applied to one of the sets of tooling. The signal from the load cell with associated controls is utilized to signal an alarm, interrupt the feeding of further extruded charges to other tooling, and stop the machine such that the affected tooling is stopped at a position for an operator to attend to the tooling.